1. Field of the Invention
This invention relates to electronic coordinate converter apparatus, particularly to harmonic oscillator coordinate converters for transforming the coordinates of a vector from a first coordinate system to a second coordinate system.
2. Description of the Prior Art
In a moving aircraft the direction from the aircraft to the target may be continually changing due to the motion of the aircraft. If a sensor having pointing means is tracking a target, the pointing direction must be continually updated with a new pointing angle in order to keep the sensor directed at the target while the airplane is moving. In such a system, the vector R is geographic coordinates X, Y, Z, with unit magnitude is continually transformed to the sensor coordinates I, J, K so that the sensor may be continually directed at a target.
In a typical avionic system, the angle through which a vector is to be transformed is in the form of a voltage and includes some noise. The noise voltage may originate mechanically or electrically at the sensor, or it may be inductively or capacitively coupled to the signal leads at some point on its way to the electronic coordinate converter. For example, the electrical noise may be random in nature due to coupling resulting in random incremental variations or fluctuations in the angle through which the vector is to be transformed. The electronic coordinate converter using a harmonic oscillator operates over a continuous range of input angles from 0 to 2.pi. radians or multiples thereof. However, within the harmonic oscillator, a harmonic oscillator oscillation angle is determined as a function of the input angle. The oscillation angle is an internal parameter which is used to control the harmonic oscillator when the vector R is rotated through an input coordinate angle. The oscillation angle is discontinuous or has a step function or transition point with respect to a particular input angle.
The harmonic oscillator is said to operate in one transformation mode for input angles on the left side of the oscillation angle control signal transition point and to operate in a second transformation mode for input angles on the right side of the oscillation angle control signal transition point.
The oscillation angle discontinuity occurs because the end of the oscillation angle is determined when one of the outputs of internal integrators crosses zero for the first time. The output of the internal integrator is either a cosine function or a sine function. A small increment in the input angle results in a small increment in the output of the integrator which may advance the sine or cosine function output beyond zero or may retard the sine or cosine function less than zero. In this event, the output of the integrator would cross zero either after an incremental oscillation or after an additional oscillation of slightly less than .pi. radius. Thus small variations in the input angle occurring in recurrent transformations may result in the oscillation angle being very small such as one milliradian and very large such as .pi. -0.001 radians. The abrupt change in oscillation angle such as from 0.001 to .pi. -0.001 may result in a slight error in the transformed vector due to the circuit components and operation. Thus, when an input angle of a vector which is continually transformed is close to the discontinuity in the oscillation angle, noise on the input angle may cause the transformation to occur on either side of the discontinuity in the oscillation angle resulting in a transformed vector with error caused by the electronic coordinate converter operating in a first and second transformation mode in addition to the noise originally on the input angle. Therefore when a sensor is directed at a target resulting in particular angular transformations across the oscillation angle transition point, the new pointing angle will have additional error due to the electronic coordinate converter operating in a first and second transformation mode. Meanwhile, for other angles on one side of the oscillation angle transition point, the new pointing angle will not have this additional source of error. The increase in error for certain angular transformations across the oscillation angle transition point due to the electronic coordinate converter operating in a first and second transformation mode is bothersome and undesirable in systems operating or tracking with high angular resolution such as 0.4 milliradians.
Thus, it is desirable to eliminate electronic coordinate converter error caused by recurrent transformations in a first and second mode (on either side of the oscillation angle discontinuity) by shifting the oscillation angle discontinuity so that recurrent vector transformations through an input angle with incremental fluctuations are performed in one transformation mode (on one side of the discontinuity).